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The Layered Earth: B2 Seafloor Spreading and Paleomagnetism Lesson B2 Seafloor Spreading and Paleomagnetism Time Required Drawing conclusion Communicating Guiding Question: Main Lesson: ideas 45 minutes Why is ocean crust so much younger than Digging Deeper continental crust? Vocabulary (optional) Seafloor spreading Interactive Magnetic reversal Animation: Curie temperature Key Concepts 5 minutes Paleomagnetism Normal polarity • The theory of seafloor spreading states that new History Link: Reversed polarity ocean crust is continually being formed, and that 5 minutes this crust is slowly carried away from its point of History Link: Assessment origin over a period of time. 5 minutes Pretest: • The study of the repeated reversal of the Contents Earth’s magnetic poles over time has provided Science Skills Brainstorming, convincing evidence of seafloor spreading. Estimating on page 244 Problem solving Measuring B-K-W-L-Q, Objective: Making inferences on page 245 To explore how magnetic studies and age information provide evidence for seafloor spreading that explains why oceanic crust is younger than continental crust. Science Background Introducing the Lesson: Seafloor Spreading Extensive mapping of the seafloor occurred during Bats use it. Dolphins use it. The commercial and WWII because submarines needed accurate data for recreational fishing industries also use sonar (from safe navigation. These studies continued after the SOund Navigation And Ranging) to locate the fish they war, and led to the discovery of ocean ridges and want to catch. The sonar systems used as part of a 1950’s ocean trenches. seafloor mapping expedition were instrumental in the discovery of the mid-oceanic ridge system, the world’s Ocean ridges: These are large oceanic mountain longest chain of mountains. At 40,000 kilometers chains with narrow central valleys extending the (approximately 25,000 miles) in length, the mid-oceanic length of an entire ocean. These mid-ocean ridge is a continuous chain of mountains running ridges connect to other ranges of mid-oceanic through every ocean around the globe. ridges that extend around the entire planet. What is the relationship between seafloor spreading Ocean trenches: These are deep-sea trenches and the world’s longest chain of mountains? How thousands of kilometers (miles) long and many does this connect with plate tectonics? Use graphic kilometers (miles) deep. They are often found at organizer B-K-W-L-Q on page 245 to compile students’ continental edges or along island chains. Most ideas, existing knowledge, and questions about ocean trenches are located in the Pacific Ocean. mid-ocean ridges and seafloor spreading. The seafloor sediment layer was much thinner than expected. None of the sediment samples were more In this lesson, students will explore the phenomenon than 180 million years old. This is very young compared of seafloor spreading and the evidence it provides to to the age of continental crust, which is measured in support the theory of plate tectonics. billions of years. www.LayeredEarth.com 69 The Layered Earth: B2 Seafloor Spreading and Paleomagnetism American geologist Harry Hess (1906–1969) explained ocean floor was comprised of parallel bands of crust the lack of old oceanic crust in 1962. Hess’ theory of having alternating magnetic polarity. These magnetic seafloor spreading states that hot magma from Earth’s bands were symmetric about the mid-oceanic ridge. mantle rises up through the mid-oceanic ridges. This magma cools and flows sideways, forming new seafloor. British scientists Fred Vine and D. H. Matthews Hot magma continues to rise from the mid-oceanic explained the phenomenon of magnetic striping in ridge, constantly producing new oceanic crust. The 1963. They suggested the magnetic striping was the older crust becomes denser as it slowly cools and signature of paleomagnetism. Their proposal stated sinks, gradually building up the mid-oceanic ridges. that magma flowing from the mid-oceanic ridges Eventually, this newly formed ocean crust is carried preserved the then-current orientation of Earth’s away from the mid-oceanic ridge by the spreading magnetic poles as the magma cooled below its Curie seafloor. It is recycled millions of years later when it temperature. Seafloor spreading carried new oceanic returns to the mantle by descending into the deep crust away from the mid-ocean ridge. Magnetic ocean trenches. Persuasive evidence to support reversals show up as bands of alternating polarity in seafloor spreading soon followed Hess’ hypothesis. the slowly spreading seafloor. The symmetric banding is the result of seafloor spreading on both sides of a mid-oceanic ridge. This explanation of magnetic Magnetic Reversal and the Curie Temperature striping by paleomagnetism convinced scientists that A compass needle points toward the north magnetic new oceanic crust was being continually formed at pole. It is little known that 800,000 years ago, the same mid-oceanic ridges. Seafloor spreading was accepted compass needle would have pointed south instead of as a reality. north. This is because the magnetic poles have reversed direction. The magnetic poles have reversed themselves Drilling and the Age of the Seafloor a number of times in Earth’s geologic history between normal polarity (i.e., the north and south magnetic Oceanic crust is younger than continental crust. Clear poles are in the orientation that they are today), and maps of seafloor age were produced in the late 1960s reversed polarity (i.e., the north and south magnetic when underwater drilling technology developed as poles are in the opposite orientation than they are part of offshore oil exploration activity. Scientific today). This is known as magnetic reversal. research vessels such as the Glomar Challenger adapted this drilling technology. Oceanic drilling Evidence for magnetic reversal comes from a physical by the Glomar Challenger between 1968 and 1983 property of magnetic materials known as the Curie produced the first accurate data of seafloor age. temperature (TC). Magnetic minerals lose their magnetism when their temperature exceeds the Glomar Challenger data showed a clear progression Curie temperature. Different magnetic materials have of increasing age with distance from the mid-ocean different Curie temperatures. As these materials cool ridge. Sediment next to the ridge was youngest. below their Curie temperature, magnetic grains in The oldest sediment was farthest away and close to the material align themselves with the magnetic continental shelves. This age progression in seafloor poles. As magnetic materials solidify, a record of their sediment corroborated the theory of seafloor magnetic orientation is “frozen” into them. Magnetite, spreading. a common compound of iron-bearing rocks like basalt, and the major component of oceanic crust, is commonly used to show this reversal. Learning Activity Paleomagnetism Students will use the software to measure the separations of mid-oceanic ridges and continental The Curie temperature provides a record of Earth’s shelves, and approximate the maximum age of the magnetic field orientation. Studies of different-aged rock seafloor. They will calculate the average rate of give information about past strength and orientation seafloor spreading and compare spread- of the Earth’s magnetic properties. This study is known ing rates as paleomagnetism. and seafloor ages on opposite sides of the mid-oceanic ridge. Students will also Paleomagnetism helped explain an unusual observation locate Earth’s greatest amount of old from the 1950s. Magnetic studies revealed that the oceanic crust. 70 www.LayeredEarth.com The Layered Earth: B2 Seafloor Spreading and Paleomagnetism Seafloor Spreading and Paleomagnetism Interactive Animation This animation shows the pattern of magnetic reversals on either side of a mid-ocean ridge as a key piece of evidence supporting the theory of plate tectonics. A geomagnetic timescale was produced by combining magnetic reversal data with the age data of the rocks (radiometric dating) in the seafloor. Learning Support Build English language ELL: Extended confidence learners vocabulary Explore NOAA’s research mission to the Galapagos Vocabulary Cards, Magnetic Spreading Center, part of the Pacific Ocean’s on page 255 mid-oceanic ridge system, at: http://oceanexplorer.noaa.gov/explorations/ 05galapagos/welcome.html DDigginigging Word Paleomagnetism DeeperDeeper Link First used in 1854, paleomagnetism is formed from the root word paleo which is derived from the ancientt Greek word palaio, meaning ancient. The word paleomagnetism literally means ancient magnetism. Did The Mariana Trench in the Pacific Ocean has a depth far greater than the height of the You 1 world’s tallest mountain. Mt. Everest is 8,848 meters (29,035 feet) high. The Mariana Know? Trench is 10,971 meters (36,000 feet) deep. Did Over 15 years, the Glomar Challenger traveled 375,632 nautical miles, investigated 624 You 2 sites, and drilled 19,119 core samples, with a total core length of 97,056 meters Know? (318,425 feet). The name Glomar is a contraction of Global Marine. History The Curie temperature is named in honor of French physicist Pierre Curie (1859–1906). In 1895, he discovered that magnetic materials lose their Link magnetic properties above a certain critical temperature. Time required: 5 minutes Travel Iceland is the only place in the world where people can visit the Link Mid-Atlantic Ridge and remain above the ocean surface. Time required: 5 minutes www.LayeredEarth.com 71 The
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